METHOD FOR BLEACHING CELLULOSE PULP FORMED FROM RECYCLED TEXTILE MATERIAL

TECHNICAL FIELD

[0001]The invention relates to a method for bleaching pulp formed from recycled textile material comprising cellulose. The invention also relates to a method for producing bleached cellulose pulp from recycled textile material and a corresponding system.

BACKGROUND

[0002]Over the recent years, recycling of textile material has gained interest. Processes for recycling cellulose from textile material comprising cellulose are known in the art, see for instance WO2018/073177A1. Recycled cellulose can be used for different purposes, for example for producing viscose. In the above-mentioned publication, the need for de-colorization/bleaching of the recycled textile material is identified, and it is suggested to use a combination of oxygen and ozone bleaching.

[0003]Methods for bleaching pulp formed from recycled textile material are known in the art. SE544236C2 proposes two-stage ozone bleaching of pulp formed from recycled textile material.

[0004]Different grades of pulp may be produced from cellulose material. In recent years, the market for low-hemicellulose pulp, or more specifically dissolving pulp, has increased. Dissolving pulp has a high alpha cellulose content, typically over 90%. Dissolving pulp is used for example to produce textiles using the viscose or lyocell processes. Such processes typically require low levels of metal ions in the cellulose pulp. For example, Fe is typically required to be 10 ppm or less. In the viscose process, Fe, Co and Mn may catalyze oxidative polymerization (ageing), and Ca and SiO₂ may be problematic in filtering and spinning. In the lyocell process, Fe and Cu may reduce the stability of the NMMO compound used as solvent.

[0005]The above-mentioned known methods for bleaching pulp formed from recycled textile material may provide cellulose pulp of sufficiently high brightness to meet the requirements of dissolving pulp, but the metal ion content may be too high to be used in for example viscose or lyocell processes. Thus, there is a need for an improved method for bleaching cellulose pulp which not only provides high brightness, but also low metal ion content.

SUMMARY

[0006]An object of the invention is to provide a method which solves or at least improves on the problems identified in the above background section.

[0007]These and other objects are achieved by the present invention by means of a method and a system according to the independent claims.

[0008]According to a first aspect of the invention, there is provided a method for bleaching pulp formed from recycled textile material comprising cellulose. The method comprises subjecting the pulp to at least one bleaching stage, and after a final bleaching stage of said at least one bleaching stage, washing and acidifying the pulp.

[0009]In other words, the method comprises bleaching the pulp in one or more (subsequent) bleaching stages, where the final bleaching stage comprises or is followed by washing and acidifying the pulp, for instance as a washing stage followed by an acidifying stage in which acidifying chemical(s) is/are added. Acidifying chemical(s) may also be added in the washing stage, i.e. the acidifying may be carried out by means of (i) adding acidifying chemical(s) to the washing, and (ii) as an acidifying stage after the washing (in which acidifying chemical(s) is/are also added). The method may further comprise, after the acidifying, washing the pulp.

[0010]It is understood that the method is a method for bleaching pulp (previously) formed from recycled textile material comprising cellulose i.e. the forming of the pulp (pulping) is not necessarily part of the method according to the first aspect of the invention. Put differently, the method comprises providing pulp formed from recycled textile material comprising cellulose.

[0011]The invention is based on the insight that the difficulties achieving sufficiently low levels of metal ions is related to that the non-standardized raw material (recycled textile material)-the quality of the textile material and the dyes used for production thereof varies greatly, and consequently so does the metal ion content. The inventors have further realized that recycled textile material may comprise buttons, zippers and other metallic parts which increase the metal content considerably. The invention is further based on the insight that by adding an acidifying stage after the bleaching, a low metal ion content can be achieved, even when the raw material varies in the above-described manner.

[0012]In embodiments, the acidifying comprises adding at least one acidifying chemical at a charge such that the pH is reduced to within a range from 1.0 to 4.0, such as 1.5 to 4.0, such as 2.0 to 3.0. It is understood that the acidifying chemical may be added in any form (as a solution, in gas phase etc.). The at least one acidifying chemical may comprise one or more of sulfuric acid (H₂SO₄), sulfur dioxide (SO₂), sodium bisulfite (NaHSO₃) and/or hydrochloric acid (HCl). For example, the acidifying may comprise adding acidifying chemicals in the form of 50 weight % HCl and 50 weight % H₂SO₄

[0013]In embodiments, the acidifying stage may be carried out at a temperature within a range from 20 to 80 degrees C. The acidifying stage may be carried out at for a duration of 2 to 60 minutes, such as 5 to 15 minutes. The acidifying stage may be carried out at pulp consistency of 2 to 14 weight % such as 5-12 weight %.

[0014]In embodiments, the acidifying stage may be carried out as part of a dilution stage comprising adding at least one chemical and dilution liquid to the pulp. The dilution stage may comprise conveying the pulp through a storage tower, a standpipe, a dilution screw and/or shredding screw and adding acidifying chemical(s) thereto.

[0015]In embodiments, the final bleaching stage may comprise hydrogen peroxide bleaching (or is a hydrogen peroxide bleaching stage), wherein the method further comprises, prior to or during the hydrogen peroxide bleaching, adding magnesium sulfate. In such embodiments, the magnesium sulfate acts as a radical scavenger to reduce a loss of viscosity due to the hydrogen peroxide bleaching. Although the addition of magnesium sulfate results in increased magnesium content in the pulp, this is at least partly handled by means of the following acidifying of the pulp. To efficiently handle the increased magnesium content as well as the metal ion content from the recycled textile material, the acidifying may, as described above, be carried out in two stages (by (i) adding acidifying chemical(s) to the washing, and (ii) as an acidifying stage after the washing in which acidifying chemical(s) is/are also added).

[0016]The pulp may be provided to the bleaching stage(s) at high consistency (HC), for example at a consistency (by weight) of at least 30%, preferably within a range from 35-42%. Alternatively, the pulp may be provided at medium consistency (MC), for example at a consistency (by weight) of 8-16%, preferably 10-12%.

[0017]In embodiments comprising two bleaching stages, the pulp may be provided to both bleaching stages at high consistency (HC), for example at a consistency (by weight) of at least 30%, preferably within a range from 35-42%. Such high consistency to the second bleaching stage may be achieved by providing pulp at high consistency to the first bleaching stage followed by the washing stage and an optional dewatering stage before the second bleaching stage.

[0018]Alternatively, the pulp may be provided to the second bleaching stage at medium consistency (MC), for example at a consistency (by weight) of 8-16%, preferably 10-12%. Such medium consistency to the second bleaching stage may be achieved by providing pulp at high consistency to the first bleaching stage followed by the washing and an optional diluting stage (or pretreating stage which comprises dilution) before the second bleaching stage. Alternatively, the pulp may be provided to the first bleaching stage at medium consistency (MC), for example at a consistency (by weight) of 8-16%, preferably 10-12%. In this case, the pulp may be provided to the second bleaching stage also at medium consistency (MC), for example at a consistency (by weight) of 8-16%, preferably 10-12%. Alternatively, the pulp may be provided to the second bleaching stage at high consistency (HC), for example at a consistency (by weight) of at least 30%, preferably within a range from 35-42%. Such high consistency to the second bleaching stage may be achieved by providing pulp at medium consistency to the first bleaching stage followed by the washing and a dewatering stage before the second bleaching stage.

[0019]In embodiments, one or more (or each) of the at least one bleaching stage comprises ozone bleaching. Preferably, at least the first bleaching stage comprises ozone bleaching. Ozone may be added as a gas mixture, for example with 8-14% ozone by weight in an ozone-air mixture. The ozone bleaching of a HC pulp may be performed for a duration of 10 seconds-5 minutes, such as 30-60 seconds, for example at a temperature of 40-70° C.

[0020]In embodiments one or more of the at least one bleaching stage comprises ozone bleaching followed by chlorine dioxide bleaching. Thus, the bleaching stage may be a ZD-stage. The chlorine dioxide bleaching may follow directly after the ozone bleaching, i.e. without intermediate washing, and may thus be referred to as a (ZD) stage. This may be advantageous since a combination of chlorine dioxide and ozone results in a synergistic effect regarding brightness level. Therefore, an addition of chlorine dioxide directly after the ozone stage without intermediate washing will reinforce the bleaching effect. Furthermore, advantageous effects in terms of viscosity control may be achieved since ozone impacts viscosity while chlorine dioxide does not. The chlorine dioxide charge may, if only one bleaching stage comprises chlorine dioxide bleaching, correspond to 30 kg active chlorine /dt or less. If two bleaching stages comprise chlorine dioxide bleaching, the total chlorine dioxide charge may correspond to 40 kg active chlorine/odt or less. The ZD-stage may be followed by a hydrogen peroxide bleaching stage, preferably with a washing stage between the i.e. chlorine dioxide bleaching and the hydrogen peroxide bleaching, i.e. such that the overall bleaching sequence is ZDP or (ZD)P.

[0021]In embodiments, one or more of the at least one bleaching stage comprises ozone bleaching followed by hydrogen peroxide bleaching. Thus, the first and/or second bleaching stage may be a ZP-stage. The hydrogen peroxide bleaching may follow directly after the ozone bleaching, i.e. without intermediate washing, and may thus be referred to as a (ZP) stage. Hydrogen peroxide is a secondary reactions product in ozone bleaching, therefore an addition of hydrogen peroxide directly after the ozone stage without intermediate washing will reinforce the ozone bleaching stage. The total hydrogen peroxide charge of the bleaching stage(s) may be less than 40 kg hydrogen peroxide/odt.

[0022]It is understood that the two embodiments described above may be combined, for example such that a first bleaching stage is a (ZD)-stage and a second bleaching stage is a (ZP)-stage.

[0023]In embodiments, one or more of the at least one bleaching stage comprises ozone bleaching and adding one or more additives, the additives comprising at least one chelating agent and/or at least one radical scavenger. Thus, the bleaching stage may be referred to as a Zq-stage. The one or more additives may be added directly after the ozone bleaching, i.e. without any intermediate washing or other step (a (Zq)-stage). The bleaching stage may comprise a dilution step (using a dilution screw for example) directly after the ozone bleaching step in which dilution liquid and the at least one chelating agent and/or at least one radical scavenger is added. The one or more additives may alternatively be added before the ozone bleaching. Chelating agents, which may also be referred to as complexing agents, provide the effect that selectivity in the ozone bleaching step is improved by reducing the content of metal ions in the pulp, which metal ions catalyze the degradation of ozone to free radicals, which in turn cause cellulose degradation. Radical scavengers have a similar effect by scavenging the metal ions. The one or more additives may be chosen from a group comprising oxalic acid, acetic acid, ethylenediaminetetraacetic acid, EDTA, diethylenetriaminepentaacetic acid, DTPA, magnesium sulfate or calcium chloride. The additives may be added at charge of 1-20 kg/odt, or more specifically if the additive is EDTA or DTPA at charge of 1-2 kg/odt. The embodiments described above may be combined, such that for example a first bleaching stage is a (Zq)-stage while a second bleaching stage is a (ZD) or (ZP) stage.

[0024]In embodiments, one or more of the at least one bleaching stage comprising ozone bleaching comprises alkaline extraction after the ozone bleaching. Thus, the bleaching stage may be a Ze-stage.

[0025]According to a second aspect of the invention, there is provided a method for producing bleached cellulose pulp from recycled textile material comprising cellulose. The method comprises pulping the recycled textile material to form cellulose pulp, for instance by swelling the material under reducing conditions as described in WO2018/073177A1. The method further comprises bleaching the formed cellulose pulp using the method according to the first aspect of the invention, or embodiments thereof.

[0026]According to a third aspect of the invention, there is provided a method for producing fibers for textile production from recycled textile material comprising cellulose, the method comprising producing bleached cellulose pulp using the method according to the second aspect of the invention or embodiments thereof and subjecting the bleached cellulose pulp to a viscose or lyocell process to produce said fibers. The viscose and lyocell processes are known in the art and will not be described in further detail here.

[0027]According to a fourth aspect of the invention, there is provided a system for producing bleached cellulose pulp from recycled textile material comprising cellulose, the system comprising a pulping arrangement arranged to receive the recycled textile material to produce pulp therefrom, at least one bleaching arrangement arranged to bleach pulp obtained from the pulping arrangement, a washing arrangement arranged to wash bleached pulp from a final bleaching arrangement of the at least one bleaching arrangement, and an acidifying arrangement arranged to acidify the washed pulp.

[0028]In embodiments, a final bleaching arrangement of said at least one bleaching arrangement is configured for hydrogen peroxide bleaching, i.e. a bleaching arrangement configured to bleach the pulp by means of adding one or more bleaching chemicals comprising hydrogen peroxide, or solely hydrogen peroxide. Such embodiments may further comprise means (such as one or more nozzles) for adding magnesium sulfate to, or prior to, the final bleaching arrangement.

[0029]In embodiments, the system further comprises means (such as one or more nozzles) for adding at least one acidifying chemical to said washing arrangement.

[0030]In embodiments, the acidifying arrangement is formed as a dilution arrangement and means for adding at least one acidifying chemical into said dilution arrangement. It is understood that the dilution arrangement also comprises means for adding at least one dilution liquid, which may comprise filtrate from one or more washing arrangements of the system. The dilution liquid and the at least one acidifying chemical may be mixed and added together to the dilution arrangement. The dilution arrangement may comprise a storage tower, a standpipe, a dilution screw, a shredder screw or combinations thereof.

[0031]The features of the embodiments described above are combinable in any practically realizable way to form embodiments having combinations of these features. Further, all features and advantages of embodiments described above with reference to the first aspect of the invention may be applied in corresponding embodiments of the second, third and fourth aspects of the invention and vice versa.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032]Above discussed and other aspects of the present invention will now be described in more detail using the appended drawings, which show presently preferred embodiments of the invention, wherein:

[0033]FIG. 1 shows a flowchart illustrating an embodiment of the method according to the first aspect of the invention, and

[0034]FIG. 2 schematically illustrates an embodiment of the method according to the third aspect of the invention, and consequently also illustrates embodiments of methods according to the first and second aspects of the invention.

DETAILED DESCRIPTION

[0035]FIG. 1 shows a flowchart illustrating an embodiment of the method according to the second aspect of the invention. The method comprises bleaching 2 pulp formed from recycled textile material. The pulp is advantageously provided in washed form or is washed prior to bleaching. The bleaching comprises ozone bleaching, i.e. bleaching by means of adding ozone to the pulp. Depending on the consistency of the provided pulp, the pulp may optionally be dewatered/thickened (not shown) prior to the bleaching 2 such that the pulp is provided to the bleaching at high consistency (at least 30%, preferably within a range from 35-42%).

[0036]After the bleaching, the pulp is washed 3 to remove dissolved material from the bleaching, whereafter it is acidified 4 by means of adding acidifying chemicals in the form of 50% HCl and 50% H₂SO₄ at a charge such that the pH is reduced to within a range from 1.5 to 4.0. The dilution/acidifying is carried out at a temperature within a range from 20 to 80 degrees C. Optionally, the pulp is cooled prior to the acidifying 4 to achieve a temperature in said range. The acidifying may be carried out for a duration of 5 to 15 minutes, and at a pulp consistency of 5-12 weight %. Depending on the consistency of the pulp received from the washing 3, the pulp is optionally diluted prior to or as part of the acidifying 4.

[0037]After the acidifying 4, the pulp is washed 5 to remove metal ion content released from the pulp due to the acidifying.

[0038]Steps 3-5 of FIGS. 1 and 2 also illustrate an embodiment of the method according to the first aspect of the invention.

[0039]FIG. 2 schematically illustrates another embodiment of the method according to the third aspect of the invention. The method comprises pulping 11 the recycled textile material to form cellulose pulp, for instance by swelling the material under reducing conditions as described in WO2018/073177A1. Optionally, the pulp is washed after pulping.

[0040]The method further comprises bleaching in two stages 12a, 12c with an intermediate washing stage 12b. In the first bleaching stage 12a, the pulp is subjected to ozone bleaching followed directly by chlorine dioxide bleaching (a (ZD)-stage). In the second (final) bleaching stage 12c, the pulp is subjected to hydrogen peroxide bleaching. As can be seen in FIG. 2, magnesium sulfate is added to the hydrogen peroxide bleaching stage 12c. It is noted that FIG. 2 is only a schematic illustration-in practice, the magnesium sulfate is usually added prior to the bleaching stage 12c, for instance in a (not shown) dilution screw.

[0041]The bleached pulp is thereafter washed 13 and then subjected to a dilution/acidification stage 14 in which recirculated filtrate from the subsequent washing 15 and acidifying chemicals are added. Optionally, or alternatively to the filtrate, dilution liquid from another source is added to the dilution stage 14. The acidifying chemicals are added in the form of 50% HCl and 50% H₂SO₄ at a charge such that the pH is reduced to within a range from 1.5 to 4.0. The dilution/acidification is carried out at a temperature within a range from 20 to 80 degrees C and for a duration of 5 to 15 minutes. Optionally, the pulp is cooled prior to the dilution/acidification stage 14 to achieve a temperature in said range. Filtrate and/or other dilution liquid are added such that the pulp has a consistency of 5-12 weight %. The dilution/acidification stage comprises conveying the pulp through a storage tower, a standpipe, a dilution screw and/or shredding screw and adding the filtrate and acidifying chemical(s) thereto. The shredding screw may be a shredding screw of a twin-roll press used in the washing stage 13, i.e. the washing stage and (part of the) dilution/acidification stage may both carried out using a single apparatus, where the pulp is diluted in the shredding screw.

[0042]Optionally, filtrate and acidifying chemicals are added (dotted line) also to the washing stage 13. Consequently, the pulp is acidified during washing 13 and diluting 14. Adding the acidifying chemicals to a shredding screw of a twin-roll press can alternatively be considered as a dilution stage after the washing. Optionally, or alternatively to the filtrate, washing liquid from another source are added to the washing stage 13.

[0043]After the dilution/acidification stage 14, the pulp is washed 15 to remove metal ion content released from the pulp due to the acidifying.

[0044]Finally, the bleached pulp (of low metal ion content) is subjected to a viscose process 16. Such processes are well known in the art and will not be shown in further detail here.

[0045]Other embodiments correspond to the embodiment in FIG. 2 except in that the first bleaching stage 12a is replaced with a Z-stage or a (Zq)-stage. Yet other embodiments correspond to the embodiment in FIG. 1 except in that the first and/or second bleaching stages are (ZP)-stages.

[0046]In the above-described embodiments, ozone is added as a gas mixture, for example with 8-16% ozone by weight in an ozone-air mixture, preferably 10-12% ozone by weight. The ozone bleaching may be performed for a duration of 10-60 seconds at a temperature of 40-70° C. The overall ozone charge of the bleaching amounts to less than 8 kg/odt, typically 5-6 kg/odt.

[0047]Stages 12a-15 of FIG. 2 also illustrate an embodiment of the method according to the first aspect of the invention. Stages 11-15 of FIG. 2 also illustrate an embodiment of the method according to the second aspect of the invention.

[0048]FIG. 2 could also be said to illustrate an embodiment of the system according to the fourth aspect of the invention, which embodiment comprises a pulping/washing arrangement 11, bleaching apparatuses 12a, 12c, washing apparatuses 12b, 13, 15 and a dilution arrangement 14 in the form of a storage tower, a standpipe and/or a dilution screw, and nozzles for adding acidifying chemicals and magnesium sulfate. Such arrangements, apparatuses and nozzles are all well-known in the art and will not be described in further detail here.

[0049]The description above and the appended drawings are to be considered as non-limiting examples of the invention. The person skilled in the art realizes that several changes and modifications may be made within the scope of the invention. For example, depending on the required brightness of the resulting pulp, one or more bleaching stages may be added prior to or after the first and second bleaching stages 12a, 12c, or part(s) of the first and second bleaching stages (for example the chlorine dioxide bleaching in stage 12a and/or the magnesium sulfate addition in stage 12c) may be omitted.